After doing some more research, I've found:

  • Imports ARE hoisted! according to the spec of ModuleDeclarationInstantiation
  • ALL the dependent Modules will be loaded before running any code.

This code will have no errors, and will work:

localFunc();

import {myFunc1} from 'mymodule';

function localFunc() { // localFunc is hoisted
    myFunc1();
}

It will be a SyntaxError. According to this part of specification:

Module :
   ModuleBody

ModuleBody :
    ModuleItemList

ModuleItemList :
    ModuleItem
    ModuleItemList ModuleItem

ModuleItem :
    ImportDeclaration
    ExportDeclaration
    StatementListItem

It means that module can contain only ImportDeclaration's, ExportDeclaration's or StatementListItem's. According to this StatementListItem could not contain ImportDeclaration nor ExportDeclaration.

import myFunc1 from 'externalModule1'; 

is an import declaration, while:

if (Math.random()>0.5) {
    import myFunc2 from 'externalModule2';
}

is a statement. So your code will result to a syntax error.

What about "will they all be loaded before running the code?". This part of specification contain next sentence:

NOTE: Before instantiating a module, all of the modules it requested must be available.

So, yeah. They will all be loaded before running the code.